A new study concludes that hundreds of young children in the District experienced potentially damaging amounts of lead in their blood when lead levels were dramatically rising in the city's tap water.

In some high-risk neighborhoods, the number of toddlers and infants with blood-lead concentrations that can cause irreversible IQ loss and developmental delays more than doubled after harmful levels of lead began leaching into the city's drinking water in 2001, according to the findings. The peer-reviewed study, obtained by The Washington Post, is to be published soon in Environmental Science and Technology, a journal on advances in chemical and environmental research.

Authors of the study, at Virginia Tech and Children's National Medical Center, said their findings raise concern about the 42,000 D.C. children, now ages 4 to 9, who were in the womb or younger than 2 during the water crisis. Those children might be at risk of future health and behavioral problems linked to lead, the report said.

The study, based on a detailed analysis of thousands of children's blood tests from 2000 to 2003, contradicts the public assurances issued by federal and D.C. health officials starting in 2004. At the time, although officials acknowledged that the amount of lead in city water were at record-breaking levels, they said repeatedly that they found no measurable impact on the general public's health.

The lead concentrations in the city's water were sometimes hundreds of times higher in individual homes than the amount the federal government considers a level of concern. The lead concentrations began rising in 2001, after a new chemical was added to the water treatment, and they persisted until they were publicized in a February 2004 Post article.

The current researchers found the most dramatic rise in the number of children with unsafe blood-lead levels in Mount Pleasant and Columbia Heights, the southeastern portion of Capitol Hill, a large swath of Ward 4 along Georgia Avenue, and Northeast Washington's Langdon Park. According to a Post analysis of lead tests in 2003 and 2004, these were all neighborhoods where some of the highest levels of lead were measured in tap water during the crisis. Those areas also relied heavily on lead pipes.

Specialists say damage from lead is often permanent. Children can exhibit signs of aggressiveness and difficulty focusing in school. Numerous studies show lead-poisoned children can on average lose 3 to 7 IQ points. The harm can be mitigated, however. Doctors say parents should focus on providing children with a healthful, calcium-rich diet and an enriching educational environment that includes reading to them regularly. There is no blood test to detect years-old lead exposure, but parents are encouraged to monitor their children.

The D.C. Water and Sewer Authority said the most recent tests of the city's water show lead levels in the safe range, with an average reading of 8 parts per billion or lower, in a group of homes. That is far below the federal level of concern of 15 parts per billion and is the result of a chemical treatment added in 2004.

Public health officials said this week that their earlier statements and reports did not try to suggest that the city's lead crisis had no impact on health but that at the time they had no data to indicate a problem. Some said they would have liked to do more research on possible links but ran into obstacles. Federal agencies and WASA cited a 2004 Centers for Disease Control and Prevention report as having "confirmed that there was no identifiable public health impact from elevated lead levels in drinking water."

The CDC relied on a broad, wide-ranging look at citywide tests for children and adults. The new study looks more specifically at children most vulnerable to lead poisoning and focused on neighborhoods with the highest levels of lead in water.

Mary Jean Brown, the CDC's lead poisoning prevention director, said she welcomes the new research and said the CDC's 2004 report noted concerns about a possible health impact but found no direct link. She said this week that she cannot control how other public agencies and officials might have interpreted the 2004 report.

"If people are walking away from this and saying there's no problem with drinking water, I don't know how to respond to that," she said. "We were under some constraints . . . working as quickly as we could. We didn't find evidence of a public health crisis. We used the methods at our disposal at the time."